1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device; and, more particularly, to a method for forming a deep trench in a semiconductor device.
2. Description of Related Art
Generally, a Bosch process proposed by Robert Bosch GmbH and a cryogenic process by etching with liquid nitrogen at a very low temperature are used to form a deep trench in a semiconductor device.
Fluorine-based plasma is used to etch a silicon substrate in the Bosch process. The Bosch process is performed by alternately performing a fluorine carbon plasma process in order to increase a mask selection rate and to protect sidewalls of the deep trench. The cryogenic process is similar to the Bosch process but the most different thing from the Bosch process is that it forms a blocking layer in the sidewalls. The blocking layer includes a stacked layer of an oxide layer in a thickness of tens of nanometers and a fluoride layer (SiOxFy).
The Bosch process and cryogenic process are proper for forming a deep trench in a Micro-Electro-Mechanical System (MEMS) in a depth of tens to hundreds of micrometers. However, since a process for forming the blocking layer and an etching process are alternately performed, a scallop phenomenon where the sidewalls are formed ruggedly is caused. If a width of the trench is broad, the scallop phenomenon may be ignored. However, if the width of the trench is narrow, an under-cut is caused in an upper portion of the trench or space void is formed due to the scallop phenomenon when the trench is buried.
FIG. 1 is a micrograph showing a cross-sectional of a conventional semiconductor device having a deep trench through one-step etching process.
As shown in FIG. 1, in a case that the one-step etching process is performed to form a deep trench, a under-cut and a void are formed in the upper portion and a sidewall of the deep trench.
Therefore, a method for forming a deep trench in a semiconductor device is developed to solve the problems caused by the scallop phenomenon.